Apparatus for electrostatic spraying of paints or similar coating materials



Dec. 12, 1967 R. GROSSTEINBECK EFAL 3,357,640

APPARATUS FOR ELECTROSTATIC SPRAYING OF PAINTS OR SIMILAR COATING MATERIALS Filed Oct. 23, 1962 INVENTORS v ROLF GROSSTEINBECK HANS HOLTZ United States Patent ABSTRACT OF THE DISCLOSURE Describes an apparatus for electrostatic sprayingof paints or similar coating material, in WhlCh the rotating spray element is surrounded by a shield which is spaced from and extends forward of the spray element so as to trap particles of the coating material dur ng the coating operation, which do not follow the electrical field lines.

The present invention relates to a process and an apparatus for electrostatic spraying of paints or s milar coating materials with the aid of a rotating spraying element and to an improvement therein which produces a more even coating over the sprayed area.

In a paint spraying apparatus of this type, as known in the art, the coating liquid is centrally fed to such a spray element. This may, for example, take the form of a hell with horizontally positioned axis having its concave s de turned toward the object to be coated, and the coat ng liquid is flung away from the edge in atomized condition under the influence of centrifugal forces. Thereafter, the fine droplets are directed onto the object to 'be coated along the lines of force maintained through a high tension electrical field between the spray element and the ob ect to be coated, by virtue of the electrical charge possessed b said dro lets.

When this process is performed with paints which contain water as the diluent, the droplets flung away from the edge of the spray element only partially follow the lines of the electrical field as these lines do not correspond to the direction of the mechanical force exerted upon the droplets. Therefore, in the case of stationary objects to be painted, an area of individual larger paint droplets is formed around the uniformly sprayed, circular paint surface. These drops, because of the thickness of paint applied thereby, have an interfering effect even if the surface portions-in the case of a moving ob ect to be painted or moving spray element-are subsequently covered with an electrostatically finely sprayed paint.

This defect in the spray behavior has heretofore stood in the way of the practical employment of water-containing paints for the electrostatic spray process by use of spray elements of the above-described type, despite the fact that water-diluted coating materials are superior to other paints because of their advantages with respect to fire and explosion dangers, savings in diluents, shipping possibilities and toxicity.

It is well known that not only the Water-containing paints but also coating materials with certain organic solvents give only poor results in commercial electrostatic s ra in a aratus.

Tlze gre nt invention, therefore, has an object to render such apparatus suitable for use even with poorly sprayable coating materials by the addition of a simple spray directional control element.

This and other objects of our invention will become apparent as the description thereof proceeds.

The process of the invention is characterized in that in apparatus for electrostatic spraying of coat ng materials comprising a rotating spray element-in COIljllllCtlOIl with the use of coating materials which contain water as the diluent or other coating materials which exhibit a poor spray behavior in such apparatus-means are provided by use of mechanical trapping devices for trapping the drops which do not follow the electrical field lines, and that the trapped liquid is again collected in a form such that it can be reused. A preferred embodiment for the performance of the process according to the present invention provides a spray element approximately coaxially surrounded by an annular surface spaced from the spray element, the edge of said annular surface nearest the object to be coated-in the direction of spray-being positioned in front of the spray element and being shaped in such a way that the trapped spray liquid is fed into a drainage device.

The accompanying drawings illustrate various embodiments of the present invention in which:

FIG. 1 is a cross-sectional view in elevation of the spray apparatus, having a stationary shield means, showing the liquid storage tank and the surface being sprayed.

FIG. 2 is a cross-sectional view in elevation showing a spray apparatus having a rotating element as part of the shield means.

FIGS. 3 and 4 show views looking into the shield means of different embodiments of the shield.

In FIG. 1, reference numeral 1 designates a known rotatable spray element in the shape of a bell into which the spray liquid is fed from the back side of a storage vessel 2 by means of pump 3, and from the spray element it is flung away from the edge by virtue of the centrifugal forces acting upon the concave side of element 1. From the edge the finely atomized and electrically charged liquid particles follow the lines of force of the high tension field indicated 'by the broken lines produced between the spray element 1 and object 4.

The coarser droplets, in contrast thereto, are either only slight influenced by the electrical field or not influenced at all and, without the additional attachment according to the present invention, would impinge upon the object to be coated outside the area sprayed with the fine particles. In order to prevent this, the spray element 1 is provided with an annular surface 5 which surrounds the spray element, as shown in cross-section in FIG. 1, and which traps the above-mentioned coarser droplets. As indicated in the drawings, the annular surface 5 is of substantially larger diameter than the diameter of the spray element 1. The edges of annular surface 5 may be provided with an inwardly extending flange 5a so that the trapped liquid can be fed into a drainage conduit 6 through which it is again returned to storage vessel 2. Annular surface 5 is connected through struts 7 to bearing housing 8 for shaft 9 of the spray element 1 and is axially movable and is fixed in a given position after adjusting it properly with respect to the spray element.

The diameter of the annular surface depends upon the diameter of the spray element. A sufficient distance should be provided between the spray element and the annular surface, to trap the drops of coating material which do not follow the electrical field lines. Most advantageously, for reasons of insulation techniques, the same electrical potential applied to the spray element is also applied at the same time to the annular surface. It should be noted that under these conditions the electrical field lines are more closely bundled together. If the potential of the object to be coated, that is, as a rule the ground potential, is applied to the annular surface, the field lines are spread further apart even if the field potential remains the same. In the event that such a change in the field pattern is undesirable under certain working conditions, it is possible to provide a suitable intermediate potential for the trapping device so that the field distribution remains unchanged by the attachment provided in accordance with the present invention.

The trapping of a portion of the paint particles flung away from the spray element merely necessitates a moderately increased rate of supply of the paint to the spray bell. This increase lies within the capacity of the supply rate increase of those types of spray apparatus which are constructed solely for use with non-aqueous paint compositions. Otherwise, an equilibrium can be achieved by a simple relative adjustment of the distance between the object to be coated and the spray bell. Upon employment of water-containing paints within the scope of the present invention, the size of the spray surface will be somewhat reduced in comparison to the corresponding surface obtained with customary spray paints. In those instances where this fact is detrimental, a compensation can be achieved by increasing the distance between the object and the spray bell. Furthermore, the size of the spray surface can be influenced by the shape of the trapping ring, as explained below.

If, under certain conditions of use, interfering droplet concentrations accumulate at the edge of the annular surface closest to the object to be coated, additional measures must be provided in order to channel the trapped liquid more rapidly to the drainage conduit. For instance, the centrifugal effect of a rotating annular surface which is conically enlarged toward the rear, may serve to lead the trapped droplets away from the edge of the annular surface closest to the object to be coated.

In FIG. 2 the arrangement of FIG. 1 is modified and uses a conically shaped trapping element 10, which is closed toward the rear. Element is rotatably mounted by means of bearing 11 on bushing 8 for shaft 9 of the spray element 1. Shaft 9 is provided with a gear 12. An internally toothed gear 14 is connected with trapping element 10. Gear 12 drives the internally toothed gear 14 by means of an intermediate gear 13. Trapping element 10 is provided with a series of apertures 15, through which the trapped liquid is flung into an annular drain 16, which is mounted by means of struts 17 on bushing 8. The drainage conduit 18 corresponds to conduit 6 in FIG. 1.

Electrostatic spraying with rotating spray elements produces a spray pattern in the form of a ring. This paint distribution, if there is a relative motion between the object and the spray element, produces a painted surface in strip form whose middle portion is covered by less paint than the two outer portions. In order not to exceed or fail to achieve the narrow limitations in this respect when water-containing paints are employed, a distribution of the centrally symmetrical spray pattern can be achieved by the trapping element within the scope of the present invention. According to FIGS. 3 and 4, the fixedly mounted trapping element, that is, especially its trapping edge, takes on an oval configuration and may be arranged with its long axis either horizontally or vertically to the direction of motion indicated in each figure by an arrow. Another modification consists in recessing the portions of the edge of the trapping ring corresponding to the central portion somewhat from the plane of this edge, so that less paint is trapped at these places.

The additional attachment according to the present invention, which is very simple in construction, makes it possible to spray with electrostatic spray apparatus of the type described even those coating materials in satisfactory manner which heretofore were indicated for other application techniques because of their poor spray behavior. This applies especially for coating materials containing water. Therefore, the present invention makes it possible to utilize the above-mentioned advantages of the material properties of these compositions as well as the advantages of electrostatic spray techniques.

While we have described certain specific embodiments and preferred modes of practice of our invention, it will be understood that this is solely for purposes of illustration and the invention is not limited thereto. Various changes and modifications may be made in the invention without departing from the spirit of the disclosure or the scope of the appended claims.

We claim:

1. 'In an electrostatic coating apparatus of the character described, a spray head for electrostatic spraying of paints and similar coating materials comprising a rotating annular spray element and an annular ring surface surrounding said spray element, said annular ring surface being of substantially larger diameter than the diameter of the spray element and said annular ring surface having the edge closest to the object to be coated situated in front of the plane of the spray element, whereby said annular ring surface traps and collects particles of coating material during the coating operation, which do not follow 1 the electrical field lines.

2. In an electrostatic coating apparatus of the character described, a spray head for electrostatic spraying of paints and similar coating materials comprising a rotating spray element and an annular ring surface surrounding said spray element, said ring surface having the edge closest to the object to be coated situated in front of the plane of the spray element wherein the annular ring surface comprises two concentric ring surfaces, of which the inner surface is rotatable and the outer surface is fixed.

3. In an electrostatic coating apparatus of the character described, a spray head for electrostatic spraying of paints and similar coating materials comprising a rotating spray element and an annular ring surface surrounding said spray element, said ring surface having the edge closest to the object to be coated situated in front of the plane of the spray element wherein the annular ring surface comprises two concentric ring surfaces, of which the inner surface is rotatable and the outer surface is fixed, drainage means from said inner to said outer surfaces, and drainage means from said outer surface.

4. In an electrostatic coating apparatus of the character described, a spray head for electrostatic spraying of paints and similar coating materials comprising a rotating spray element and an annular ring surface surroundsaid spray element, said ring surface having the edge closest to the object to be coated situated in front of the plane of the spray element wherein the annular ring surface comprises two concentric ring surfaces, of which the inner surface is rotatable and the outer surface is fixed and means to rotate said inner surface.

5'. In an electrostatic coating apparatus of the character described, a spray head for electrostatic spraying of paints and similar coating materials comprising a rotating spray element and an annular ring surface surrounding said spray element, said ring surface having the edge closest to the object to be coated situated in front of the plane of the spray element wherein the annular ring surface comprises two concentric ring surfaces, of which the inner surface is rotatable and the outer surface .is fixed and means to rotate said inner surface, said means being operated by said rotating spray element.

References Cited UNITED STATES PATENTS 2,754,226 7/1956 Juvinall 11793.44 2,784,114 3/1957 Miller 1l7--93.41 X 2,976,175 3/1961 Reindl l1793.44 X

ALFRED L. LEAVITI, Primary Examiner.

A. GOLIAN, Examiner. 

1. IN AN ELECTROSTATIC COATING APPARATUS OF THE CHARACTER DESCRIBED, A SPRAY HEAD FOR ELECTROSTATIC SPRAYING OF PAINTS AND SIMILAR COATING MATERIALS COMPRISING A ROTATING ANNULAR SPRAY ELEMENT AND AN ANNULAR RING SURFACE SURROUNDING SAID SPRAY ELEMENT, SAID ANNULAR RING SURFACE BEING OF SUBSTANTIALLY LARGER DIAMETER THAN THE DIAMETER OF THE SPRAY ELEMENT AND SAID ANNULAR RING SURFACE HAVING THE EDGE CLOSEST TOT HE OBJECT TO BE COATED SITUATED IN FRONT OF THE PLANE OF THE SPRAY ELEMENT, WHEREBY SAID ANNULAR RING SURFCE TRAPS AND COLLECTS PARTICLES OF COATING MATERIAL DURING THE COATING OPERATION, WHICH DO NOT FOLLOW THE ELECTRICAL FIELD LINES. 